This application describes a five-year mentored training program to develop an independent clinician-scientist whose career in academic medicine will focus on furthering our understanding of human red blood cell development. Beta-thalassemia in humans is a disease associated with anemia, splenomegaly, and ineffective erythropoiesis; iron overload in this disease results from transfusion and increased iron absorption from hepcidin deficiency. Our lab previously demonstrated that treatment with transferrin in mice with beta- thalassemia improves the disease phenotype with reversal of anemia, splenomegaly, and ineffective erythropoiesis and an increase in hepcidin expression. Surprisingly, although there are more red cells in circulation, their size and hemoglobin content is reduced in treated mice. We hypothesize that supplemental transferrin results in more iron deficient erythropoiesis, reducing the imbalance between heme, alpha-globin, and beta-globin synthesis in beta-thalassemia. My goal in the studies proposed for this award is to test this hypothesis. I plan two specific aims to evaluate this: 1) to determine if iron uptake by erythroid precursors is altered when additional transferrin is available in vivo and in vitro; and 2) to determine if the regulation and production of hemoglobin components as well as iron and heme export is altered when supplemental transferrin is added in normal and beta-thalassemic cells in vivo and in vitro. These studies will test my hypothesis that exogenous transferrin restricts iron entry into erythroid precursors and protects early erythroid cells from heme toxicity, will provide insight into the physiology responsible for the phenotype in beta- thalassemia, and will further our understanding of normal and disordered erythropoiesis. I will accomplish these specific aims by the following methods: 1) characterize the ferrokinetics of erythroid precursors in mice given additional transferrin, analyze the expression of transferrin receptor in these cells in vivo, and evaluate cytosolic iron uptake in cell lines and human cells in culture; 2) evaluate expression levels and regulators of heme and globin production, measure levels of zinc protoporphyrin and free heme in the cytosol, and characterize the effect on FLVCR and FPN-1B, heme and iron exporters, respectively, during erythroid precursor differentiation in mouse bone marrow and spleen samples as well as in human samples in culture. These studies focus on basic red blood cell physiology and may lend insight into human diseases where red blood cell development is disturbed. Lastly, progress in understanding mechanisms of transferrin efficacy in beta-thalassemic mice may enable its therapeutic development for patients with this disease.